Most telephone networks in use today utilize hierarchical call routing. FIG. 1 illustrates a conventional hierarchical telephone network 10. The hierarchical network 10 comprises a plurality of nodes in the form of the central offices A, B, C, D, E, F, G, H, I. These central offices may be implemented by switches such as the #1A ESS. The central offices A, B, C, D, E, F, G, H, I are connected by the trunks 11. Individual units of customer premises equipment at individual subscriber locations are connected to particular central offices via a subscriber loop. For example, the customer premises equipment at the subscriber location 12 is connected via the subscriber loop 14 to the central office A. Similarly, the customer premises equipment at the subscriber location 16 is connected to the central office I via the subscriber loop 18.
Consider the case where the subscriber location 12 desires to place a call to the subscriber location 16. The best route through the network 10 is determined, e.g., a route through the central offices A, E, F, and I, and the call is completed.
In the United States, each subscriber location has a telephone number comprising an area code (three digits) a local exchange or NNX number (three digits) and a subscriber line number (four digits). Consider the case where the subscriber location 16 of FIG. 1 has the area code "abc", the NNX number "ghi" and the subscriber line number "uvxy". In a telephone network such as the network 10 of FIG. 1, the routing is hierarchical. A call originating at the subscriber location 12 and destined for the subscriber location 16 is transmitted to the local central office A via the subscriber loop 14. The central office A routes all calls it receives with the area code "abc" to the central office E so that the central office A is transparent to everything in a called telephone number that follows the area code "abc". Similarly, the central office E routes all calls with the area code "abc" to the central office F so that the central office E is also transparent to everything in a called telephone number which follows the area code "abc". The central office F routes all calls with an NNX number "ghi" to the central office I so that the central office F is transparent to the subscriber line number which follows the NNX number "ghi". Then, the central office I routes the call via the subscriber loop 18 which has the line number "uvxy" to the subscriber location 16. Because of this hierarchical routing, all of the telephone numbers with a particular NNX number (i.e., all of the telephone numbers of a particular local exchange) are connected to the same local central office. If telephones having the same NNX were connected to different central offices, a hierarchical routing scheme could not be utilized.
The hierarchical network 10 of FIG. 1 has a large number of central offices. In the past approximately fifteen years, transmission costs in telecommunication networks have been reduced by about three orders of magnitude. On the other hand, reductions in switching costs have been much more limited. Switching costs have been reduced by about a factor of four, i.e, less than one order of magnitude. In view of the large drop in transmission costs and the much more modest drop in switching costs, the efficiency of the conventional hierarchical telephone network is not optimal because a large number of central office switches is utilized to minimize the transmission distances for particular connections. Presently, in the United States there are about 10,000 central office switches and the number of lines on a switch is about 20,000.
In the conventional hierarchical network, telephone number retention is generally impractical. If a subscriber location with a specific NNX number in its telephone number were to be moved from the central office normally associated with the specific NNX number to a new central office, it would be necessary to update the routing tables of all central offices located within one hop of the original central office and within one hop of the new central office.
The lack of telephone number retention impacts the introduction of new services into the conventional hierarchical network. New services include for example digital voice, video and real time data transfer. Such services can be provided by upgrading a network to ISDN. In order to make ISDN available for all subscriber locations attached to a hierarchical network, it is necessary to upgrade simultaneously a large number of central offices to ISDN standards. If only a few of the central offices in the network are upgraded, only subscriber locations having local exchanges (i.e. having NNX numbers) belonging to the upgraded central offices will be able to take advantage of ISDN. Other subscriber locations with other NNX numbers will either have to change the central office to which they are connected and, therefore, also, change their telephone numbers, or else forego the use of the new telecommunication services. The changing of a telephone number can be quite costly to a business and is something both businesses and individuals will strive to avoid.
When a new service is first offered to the public, demand for the new service will generally be low, and the revenue that the new service will generate for the telephone companies is limited. On the other hand, there is a need for the telephone companies to spend large amounts of money to upgrade numerous central offices to make the new service widely available. If only a few central offices are upgraded, a new service may never be successful because it cannot be offered to a large enough body of potential customers. In addition, customers who do subscribe will be able to use the new service to communicate only with other customers who are also connected to an upgraded central office.
In view of the foregoing, it is an object of the present invention to provide a telecommunication network in which a subscriber location can change the node or central office to which it is connected without changing its telephone number, i.e., it is an object of the invention to provide a network with telephone number retention. More particularly, it is an object of the present invention to provide a telecommunication network which permits new services, such as ISDN, to be introduced to a large number of subscribers, but wherein only a limited number of nodes or central offices need to be upgraded to offer the new service. It is also an object of the invention to provide an architecture for a telecommunication network wherein subscribers not directly connected to an upgraded central office can access the upgraded central office and take advantage of new services without changing their telephone numbers.
One prior art network architecture which takes advantage of the large reductions in transmission costs by utilizing a relatively small number of well-placed switches is a ring network which makes use of the SONET (Synchronous Optical Network) signal transmission techniques. Another network which utilizes a small number of switches is a linear add/drop network which makes use of the SONET signal transmission techniques. Before discussing these networks, it will be helpful to briefly review the SONET signal transmission techniques. As used herein, the term SONET refers to a family of digital signals whose bit rates are integer multiples of a basic module signal. (See, e.g., R. Ballart and Yau-Chau Ching, "SONET: Now It's the Standard Optical Network," IEEE Comm. Magazine, March 1989, pp. 8-15; H. Sabit Say and R. Young, "SONET: Optical Highway of the 1990s and Beyond," Bellcore Exchange, July/August 1988, pp. 3-7;J. E. Jakubson, "Managing SONET Networks," IEEE LTS, November 1991, Vol. 2, No. 4, pp. 5-13; H. Shirakawa, K. Maki, and H. Miura, "Japan's Network Evolution Relies on SDH-Based Systems," IEEE LTS, November 1991, Vol. 2, No. 4, pp. 14-18; Michael To and James McEachern, "Planning and Deploying a SONET-based Metro Network," IEEE LTS, November 1991/Vol. 2, No. 4, pp. 19-23; T. R. Eames and G. T. Hawley, "The Synchronous Optical Network and Fiber in the Loop," IEEE LTS, November 1991, Vol. 2, No. 4, pp. 24-29; Izaz Haque, W. Kremer and K. Raychaudhuri, "Self-Healing Rings in a Synchronous Environment" IEEE LTS November 1991, Vol. 2 No. 4, pp. 30-37; P. Passeri, F. Balena, G. Bars, N. Vogt, and T. Wright, "Introducing SDH Systems in Europe," IEEE LTS, November 1991, Vol. 2, No. 4, pp. 38-43; Yau-Chau Ching and Grant W. Cyboron, "Where is SONET?" IEEE LTS, November 1991, Vol. 2, No. 4, pp. 44-51; C. N. Day and Chi-Ho Lin, "SONET and OSI: Making a Connection," IEEE LTS, November 1991, Vol. 2, No. 4, pp. 52-59.)
The basic module or first level of the SONET signal is called the Synchronous Transport Signal-Level 1 (STS-1). The STS-1 has a bit rate of 51.84 Mb/sec and is synchronous. The STS-1 signal is formed from a sequence of repeating frames. The STS-1 frame structure is illustrated in FIG. 2. The STS-1 frame structure can be drawn as 90 columns by 9 rows of 8-bit bytes. The order of transmission of the bytes is row by row, from left to right across the columns, with one entire frame being transmitted every 125 .mu.s. The 125 .mu.s frame period supports digital voice signal transport encoded using 1 byte/125 .mu.s=64 kb/s. The first three columns of the STS-1 frame contain section and line overhead bytes. The remaining 87 columns form the STS-1 Synchronous Payload Envelope (SPE). The SPE carries SONET payloads including 9 bytes of path overhead. The STS-1 can carry a clear channel DS3 signal (44.736 Mb/s) or, alternatively, a plurality of lower-rate signals such as DS0, DS1, DS1C, and DS2 by dividing the Synchronous Payload Envelope into a plurality of fixed time slots. For example, 648 DS0 signals fit into the SPE of an STS-1 signal.
Higher rate SONET signals are obtained by byte interleaving N frame aligned STS-1 signals to form an STS-N signal in accordance with conventional SONET technology. An STS-N signal may be viewed as having a repetitive frame structure, wherein each frame comprises the overhead bits of N STS-1 frames and N synchronous payload envelopes. For example, three STS-1 signals may be multiplexed by a multiplexer into an STS-3 signal. The bit rate of the STS-3 signal is three times the bit rate of an STS-1 signal and the structure of each frame of the STS-3 signal comprises three synchronous payload envelopes and three fields of overhead bits from the three original STS-1 signals. When transmitted using optical fibers, the STS-N signal is converted to optical form and is designated as the OC-N signal.
FIG. 3 shows a ring network which utilizes the SONET signal transmission technique. The ring network 100 of FIG. 3 comprises a plurality of nodes 102, 103, 104, 105, 106, 107. Each node 102, 103, 104, 105, 106, 107 comprises an add/drop (A/D) multiplexer 112, 113, 114, 115, 116, 117. The central offices 122, 123, 124, 125, 126, 127 are connected to the A/D multiplexers 112, 113, 114, 115, 116, 117, respectively.
Illustratively, the central offices 122, 123, 124, 125, 126 are local central offices and a large number of subscriber locations are connected to these central offices. For example, a unit of customer premises equipment at the subscriber location 130 is connected to the central office 123 via the subscriber loop 131. Similarly, the units of customer premises equipment at the subscriber locations 132 and 133 are connected to the central office 124 via the subscriber loops 134 and 135. In addition, a unit of customer premises equipment at the subscriber location 138 is connected to the central office 126 via the subscriber loop 139. The central office 127 is a tandem central office for switching calls between the ring network 100 and the hierarchical network 150. The hierarchical network 150 comprises the central offices 127, 151, 152, 153, 154, 155. Illustratively, a unit of customer premises equipment at a subscriber location 160 is connected to the central office 151 via the subscriber loop 161.
The operation of the ring network 100 of FIG. 3 is as follows. A SONET signal comprising frames divided into time slots circulates around the ring 110. Typically, the SONET signal on the ring network 100 of FIG. 3 is a 3.2 Gigabit/sec STS-64 signal which contains 32,000 DS0 slots in each frame. In some cases the SONET ring network is bidirectional, in which case a SONET signal circulates in both directions around the ring 110.
The frames are received at an input of each A/D multiplexer (e.g. the input 129 of the A/D multiplexer 112) and regenerated at the output of each A/D multiplexer unit (e.g. the output 130 of the A/D multiplexer 112). Each A/D multiplexer can write data received from a subscriber location via central offices into particular time slots for transmission to a remote A/D multiplexer. For example, the central office 122 inputs data into the A/D multiplexer 112 via the input 128 and the A/D multiplexer 112 in turn writes this data into particular time slots in the SONET signal circulating around the ring. Each A/D multiplexer can also read data out of particular time slots originating from a remote A/D multiplexer and destined for an associated unit of customer premises equipment. In each SONET frame, particular time slots can be permanently dedicated to logical connections between particular pairs of nodes. For example, slot #1, slot #3 and slot #17 in each frame may be dedicated to logical connections between node 102 and node 104. Similarly, slot #2, slot #4 and slot #18 of each frame may be dedicated to logical connections between the node 103 and the node 107. The use of the dedicated time slots in the SONET ring network is utilized to achieve logical connections between pairs of nodes and between pairs of central offices. In contrast, in a hierarchical network, central office pairs are physically connected by trunks.
When the subscriber location 130 wishes to connect to the subscriber location 132, the call is routed via the subscriber loop 131 to the central office 123. The central office 123 looks at a prefix (i.e. area code and/or exchange number) contained in the called telephone number and determines that the call is to be routed to a specific node on the ring, in this case the node 104. The central office 123 maintains a table which indicates which slots in the SONET frame are dedicated to logical connections between the node 103 and the node 104. The A/D multiplexer 113 writes data to be transmitted from the node 103 to the node 104 into time slots in the SONET frames dedicated for a logical connection between this pair of nodes. This data is read out of the dedicated time slots by the A/D multiplexer 114 at the node 104 and transmitted to the central office 124. The data is then routed by the central office 124 via the subscriber loop 134 to the subscriber location 132. Through use of the dedicated time slots, the network may be viewed as forming a "logical trunk" between the node 103 and the node 104.
In another example, consider a call originating at the subscriber location 160 in the hierarchical network 150 and destined for the subscriber location 138 in the SONET ring network 100. The call is routed through the hierarchical network 150 and is received at the central office 127. The central office 127 looks at a prefix in the called telephone number to determine that it needs to utilize a time slot dedicated for transmitting from the node 107 to the node 106. The A/D multiplexer 117 writes data into the appropriate time slot in the SONET frame to establish the call connection to the node 106 and its central office 126.
Instead of utilizing a ring configuration, the A/D multiplexers may be arranged in a linear add/drop network and a SONET signal can be synchronously transmitted in both directions from one end of the linear network to the other.
While a conventional SONET ring or linear add/drop network may be utilized to reduce the number of central office switches, a conventional SONET network, such as the network 100 of FIG. 3, does not provide a practical way of implementing telephone number retention. In a SONET network calls are routed to destination nodes and destination central offices based on a prefix (i.e., area code or NNX) contained in a called telephone number. Therefore, the only way to provide telephone number retention in the conventional SONET ring network is to update simultaneously the routing tables of all of the central offices, an approach which is not practical. As a result, it is still necessary to upgrade all of the central offices to provide new services to all subscriber locations. If only some of the central offices are upgraded, subscribers who are not connected to an upgraded central office and who desire the new services will have to be reconnected to an upgraded office and, therefore, will have to change their telephone numbers.
In view of the foregoing, it is a further object of the present invention to provide an improved SONET network which has telephone number retention. In addition, it is an object of the invention to provide an improved SONET network in which new services can be made available to a subscriber who is not directly connected to an upgraded node or central office, without the subscriber having to change its telephone number. It is also an object of the present invention to provide an improved SONET network wherein new services can be offered to a large body of potential subscribers, while only upgrading a limited number of central offices.